The efficient conversion of light hydrocarbon gases, such as natural gas, and carbon dioxide into high quality syngas has several commercial and financial advantages:
A) Some natural gas or light hydrocarbon resources can't be economically recovered since the local infrastructure is not adequate to economically transport this gas to commercial customers. These resources are typically referred to as “stranded resources”.
B) Natural gas resources can contain 2-50% (or higher) carbon dioxide which needs to be removed at the extraction site before commercial use.
C) Natural gas resources contain varying amounts of C2-C6 hydrocarbons which needs to be removed at the extraction site or from the natural gas pipelines before commercial use of the natural gas.
D) Many other processes (e.g. power plants, cement plants, ethanol production, petroleum refining, chemical plants, etc.) produce carbon dioxide which is usually discharged into the atmosphere. Since carbon dioxide has been identified as a significant greenhouse gas, these carbon dioxide emissions need to be reduced from these processes. Although, this carbon dioxide can be used to enhance oil and gas recovery from wells in limited cases, the majority of this captured carbon dioxide will be emitted into the atmosphere. Since carbon dioxide is a carbon containing gas, the preferred method is to efficiently capture the carbon dioxide and convert it to fuels (e.g. diesel fuel) and chemicals.
The conversion of light hydrocarbon gases into more valuable chemical products typically involves syngas generation. Syngas generation involves converting natural gas, which is mostly methane, to syngas, which is primarily a mixture of carbon monoxide and hydrogen. Syngas may be used as a feedstock for producing a wide range of chemical products, including liquid fuels, alcohols, acetic acid, dimethyl ether and many other chemical products. However, this syngas needs to be directly produced and converted at the resource site to fuels and/or chemical products since it is not practical to transport the syngas to distant refineries and chemical processing plants.
There are a few possible approaches to converting remote natural gas assets into syngas. Several catalysts are commercially available to convert natural gas into syngas. The syngas produced has a H2/CO ratio that varies from 3.0-4.5/1.0. However, the H2/CO ratio needs to be in the proper stoichiometric range of 1.5-2.5/1.0 for the production of fuels and chemicals. Unless otherwise stated, syngas ratios (and percentage compositions) as described herein are in terms of molar ratios (and molar percentages).
Since the syngas generation is a potentially costly step, it is important to produce syngas with the desired H2/CO ratio for the subsequent production of the desired products. Therefore, several alternative processes for syngas generation have been developed.
One alternative process for syngas generation involves the catalytic or thermal reforming reaction between carbon dioxide and methane (typically referred to as dry reforming). An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reaction. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0.
A second alternative process for synthesis gas generation involves partial oxidation of methane using oxygen, where the oxygen can be either air, enriched air, or oxygen with a purity in excess of 90%, preferably in excess of 99%. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0. Likewise it is common to add relatively small amounts of CO2 to the feed gas mixture in an attempt to adjust the ratio closer to 2.0.
A third approach is to produce syngas with a H2/CO ratio between 0.5 and 1 using a mixture of LPG and CO2 (Calcor process). See, Hydrocarbon Processing, Vol. 64, May 1985, pp. 106-107 and “A new process to make Oxo-feed,” Hydrocarbon Processing, Vol. 66, July 1987, pg. 52. However, many natural gas resource sites, in particular the stranded natural gas sites, do not have the infrastructure available to separate LPG and CO2 from the natural gas.
Many processes and catalyst formulations have been reported in the literature for the reforming of light hydrocarbon gases or carbon dioxide. In the first step in the process, the production of syngas traditional catalysts do not meet the following criteria: 1) exhibits high thermal stability up to 1,100° C.; 2) does not produce elemental carbon (coking); 3) has good resistance to contaminants that may be present in captured CO2 and natural gas streams; 4) can be reduced in-situ in the catalytic reactor; 5) exhibits good physical hardness and will not physically degrade over time; 6) will efficiently co-convert CH4 and CO2, with and without the presence of water.
It is possible to produce syngas with a H2/CO ratio that is above the ratio ideally desired for the process in which the syngas is to be used, and then to remove excess hydrogen to adjust the ratio to the desired value. However, the H2 removal process employs expensive H2 separation systems that tend to foul and decline in performance with use.
Some natural gas extraction plants produce LPG as well as the natural gas. The export of LPG from such a facility or from the parent natural gas field is often difficult and expensive. The LPG must be compressed or liquefied, and the shipment requires the use of special transportation vessels. Furthermore, the market for mixtures of propane and butane is limited and of reduced value. Thus, the LPG must be separated into individual propane and butane of sufficient purity to meet commercial specifications. This complicated and expensive operation often results in high costs, which limits the value of the LPG at the production site.
The conversion of natural gas to liquid fuels further involves the production of some quantities of greenhouse gas emissions, such as CO2, which is environmentally undesirable.
Following the production of the synthesis gas, many processes and catalysts have been proposed for the production of transportation fuels and chemicals. However, the traditional process for production of fuels and chemicals from syngas involves the production first of a paraffinic wax product that is then refined into fuels and/or chemicals. The refining step is capital intensive and complex to operate, therefore requiring large plant sizes to justify this refining system.
Accordingly, there is a need for a process for producing a syngas with a pre-selected H2/CO ratio that can be varied according to the process in which the syngas is to be employed and that avoids H2 separation and coking in the syngas formation step. There is also a need for a process that minimizes or eliminates production of LPG from a processing facility, such as, for example, a hydrocarbon synthesis facility. Furthermore, there is a need to reduce the greenhouse emissions from a processing facility, such as, for example, an on-site fuel production plant. In addition, the need to directly produce a usable diesel fuel without having to refine a hydrocarbon wax is required to justify lower plant capital and operating costs.